The present invention relates to a method of manufacturing an oil seal in which a sliding surface of a rotary shaft consists of different elastomeric materials having different moduli of elasticity or hardnesses so that a hydrodynamic pumping action is provided by making use of the deflection of a sealing surface that is caused by rotation of the rotary shaft.
FIG. 1 shows an example of a conventional oil seal of this kind, as disclosed in U.S. Pat. No. 4,300,777. In this prior art reference, an oil seal generally designated at 100 has a metallic ring 100a having a generally L-shaped section, and a sealing lip 101 which is made of an elastomeric material is integrally bonded to the radial inner edge of the metallic ring 100a and extends to a sealed liquid side (i.e., to the right side as viewed in FIG. 1). The sealing lip 101 has a lip tip portion 102, and a side surface which is on the sealed liquid side of the lip tip portion 102 is provided with a different material layer 103 made of an elastomeric material which is different from the material of the sealing lip 101. Thus, a sliding surface 105 which is in sealing contact with a shaft 104 is formed by first and second sliding surfaces 106 and 107 which are made of different rubbery materials. More specifically, the first sliding surface 106 is made o an elastomeric material having a lower modulus of elasticity than that of another elastomeric material employed for the second sliding surface 107. Therefore, when the shaft 104 is rotating, the sliding surface 105 is pulled in the direction of rotation of the shaft, with each of the first and second sliding surfaces 106 and 107 being deflected from the axial direction to a helical angle of 0, as shown in FIG. 2, due to the difference in modulus of elasticity. This deflection gives a certain inclination to fine projections on the surface of the sliding surfaces 106 and 107 to provide a hydrodynamic pumping action.
With respect to methods of manufacturing an oil seal of this kind, it is possible to adopt a method comprising molding the sealing lip 101 in a known manner and thereafter providing the different material layer 103 on the sealed liquid side surface of the lip tip portion 102, for instance, by bonding. This method, however, may result in an increase in the number of processes. In order to obtain a reduction in the number of processes, it is possible to adopt another method comprising vulcanization molding the sealing lip 101 and a different elastomeric material within a mold to form an integral structure. An example of this method of vulcanization molding different materials within a mold to form an integral structure is the method disclosed in Japanese Patent Publication No. 23681/1971, which is shown in FIGS. 3 and 4. This prior art discloses a method of molding a layer of a fluorine-containing-resin on a sliding surface of a sealing lip to form an integral structure, which comprises the steps of placing an elastomeric material 111 and a fluorine-containing-resin 112 within a mold 110 in a superposed condition, compression molding the material 111 and the resin 112 to form an integral structure in which a fluorine-containing-resin layer 112a is provided on the atmospheric side surface of the lip tip portion 102, and, after the compacted structure has been taken out of the mold, cutting, along the line C--C shown in FIG. 4, a surplus portion 113 that extends to the sealed liquid side of the lip tip portion 102.
Although this conventional method enables different materials to be molded to form an integral structure, the method means that the sliding surface 105', which is formed by cutting the surplus portion 113 after vulcanization molding, is formed solely by a layer 103' of the second material, i.e., the resin, as shown in FIG. 5, thus failing to obtain a sliding surface 105' formed of two different materials. In order to compensate for this drawback, the lip tip portion 102 may be cut in the axial direction along a line D--D shown in FIG. 5 by means of a knife or scalpel so that the sliding surface 105' will be formed of two different materials. This operation, however, requires highly precise control of the amount to be cut off from the lip tip portion 102 in order to prevent any variation in the respective widths W1 and W2 of the sliding surfaces 106' and 107', particularly a variation in the width W1 of the sliding surface 106' on the sealed liquid side, because such a variation may cause variation in the deflection amount of the sliding surface during rotation of the shaft when the oil seal is used, and accordingly, variation in the hydrodynamic pumping action. In this respect, it is difficult to form the sliding surfaces properly.
Even if the the sliding surface 106' on the sealed liquid side is formed to have a predetermined value of the width W1, this width W1 may increase after a long period of use as wear of the surface proceeds. This increase in the width W1 of the surface on the sealed liquid side causes an increased amount of deflection during rotation of the shaft, resulting in an increase in the degree of suction to which an external fluid is subjected by the hydrodynamic pumping action and accordingly insufficient lubrication of the sliding surface 105', hence leading to various problems including seizure.